1. Field of the Invention
This invention relates to a connection control device for controlling the connection of a small generator unit installed at an electric power consumer to an electric power system.
2. Description of the Related Art
Recently, small generators of the stationary inverter type in which DC power of a solar battery or fuel battery is converted into AC power with an inverter, and small generators of the co-generation type in which an engine and a generator is combined to produce AC power and heat, have been proposed to utilize natural energy and to use energy effectively. The small generator is connected to an electric power system (including a power plant, transmission lines, substations, distribution lines, etc.), so as to utilize the power of the latter when the small generator becomes out of order, or when the electric power generated by the small generator is insufficient to drive the load. That is, the connection of the small generator to the electric power system ensures the stable supply of electric power to the load.
However, the connection of the small generator to the electric power system may cause the following problem: Sometimes installation or repair of an electric power system is carried out with the breaker opened; i.e., with the supply of electric power suspended. If, in this case, a small generator is connected to the electric power system, then the voltage of the small generator is applied to the distribution line (so-called "reverse voltage" is applied to the distribution line), although the electric power system has been interrupted with the breaker opened so as not to supply electric power to the distribution line connected thereto. Because of the reverse voltage, persons engaging in the installation or repair of the distribution line may be electrocuted.
In order to eliminate the difficulty, heretofore the following method is employed: An under- and over-voltage relay (reference voltage.+-.about 15%) and an abnormal frequency relay (reference frequency.+-.about 1 Hz) are provided for the small generator to detect the voltage and frequency provided when the electric power system is interrupted. When the difference between the voltage thus detected and the reference voltage exceeds the tolerance, or when the difference between the frequency thus detected and the reference frequency exceeds the tolerance, the relay determines that the electric power system has been interrupted, and operates to disconnect the small generator from the electric power system.
However, the method is disadvantageous in that, when the electric power outputted by the small generator is in balance with the load to be handled by the electric power consumer, the voltage and frequency change slightly, so that the relays cannot detect the variations, and accordingly the interruption of the electric power system cannot be detected.
In order to eliminate the above-described difficulty, the inventors have proposed the following connection control device in the art (Japanese Patent Application (OPI) No. 237327/1988 (the term "OPI" as used herein means an "unexamined published application"), entitled "Connection Control Device for Controlling the Connection of Dispersed Power Sources to Electric Power System"). The device proposed is based on the fact that it has been found through comparison of the frequency variations (or fluctuations) of the electric power system and the small generator that the frequency variation of the electric power system has a period inherent in the electric power system, whereas the frequency variation of the small generator with an inverter unit is free from that inherent in the electric power system. That is, the frequency variation is monitored at the small generator, and when the frequency variation inherent in the electric power system disappears, it is determined that the supply of electric power of the electric power system has been suspended, and the small generator is disconnected from the electric power system.
The inventors have conducted intensive research on the connection control device, and found that the invention proposed under Japanese Patent Application (OPI) No. 237327/1988 is still disadvantageous in the following point: In the case where the small generator is made up of a stationary generator such as a solar generator and a rotary generator of co-generation type with a diesel engine, a frequency variation may occur which is similar to that inherent in the electric power system, which makes it difficult to determine whether or not the electric power system has been interrupted.
In view of the foregoing, an object of this invention is to provide a connection control device for controlling the connection of a small generator to an electric power system with which, the interruption of the electric power system can be detected even in the case where a small generator such as a rotary generator of co-generation type with a diesel engine is employed.